At present, these specific substrates can be fabricated (and sold) by molecular bonding of two substrates one to the other, followed by thinning, for example by grinding, of one of them (these processes include BSOI (bonded SOI), BESOI (bonded and etched-back SOI), and Eltran (epitaxial layer transfer). These processes have the drawback of consuming two substrates in order to fabricate only one (one of the substrates being for the most part consumed during the thinning step).
In parallel with this, the Smart Cut™ process is a process for fabricating standard SOI substrates (using silica as the insulator with a thickness exceeding 50 nm), which is predominant at present for reasons as much of cost as of the quality of the material. In summary, this Smart Cut™ process consists, for example, in implanting a given species at a given depth in a substrate on the surface of which is formed a layer of oxide forming an insulator, and bonding a second substrate onto the surface provided with the insulator layer. This bonding is often molecular bonding, advantageously followed by consolidation heat treatment. A fracture is then produced at the level of the implanted layer, in practice at least in part by heat treatment, so that a substrate is obtained (the aforementioned second substrate) to which is bonded a thin layer of the aforementioned first substrate, hugging the layer of insulator.
However, the various known approaches have various limitations.
Thus the Smart Cut™ process includes a fracture step, necessary for separating the SOI substrate from the rest of the donor substrate, which step does not exist in the other processes (BSOI, BESOI, Eltran, etc.); it is true that this additional step enables reuse of the rest of the donor substrate, unlike other processes in which thinning consumes the whole of one of the substrates, except for the film forming part of the final SOI structure. However, this specific step of the Smart Cut™ process can create specific defects if particular precautions are not taken, for example in the case of certain thin insulators, if water is trapped in the bonding step.
In practice, the insulator of an SOI substrate can have thickness variations over the substrate; the final product can conventionally include various patterns whose specific characteristics depend on the specific future role of these patterns (especially where the insulator is concerned). At present, the fabrication of a substrate whose insulator is not of uniform thickness is determined by the subsequent use that will be made of it: the differences between the insulators of the various patterns are created during the fabrication of the substrate, according to a scheme determined by the end product. This specificity limits the versatility of the substrate (from the fabrication stage itself, it is no longer usable except in the target application, which in practice prevents fabricating substrates without knowing their intended use beforehand) and introduces, at the substrate fabrication stage, photolithography and etching steps (usually reserved for the production of integrated circuits) that constrain substrate fabricators to increase their investment in hardware and technological know-how in order to be able to carry out these steps. Moreover, the users of the substrates must share information on the use of their products with their substrate suppliers, which can compromise confidentiality in respect of the intended use of the substrates ordered from those suppliers.